Well, trainable deck torpedo tubes were used on many of French pre-war submarines (and in some other navies, like Danish & Polish). The idea was to aim the torpedoes without turning the whole submarine & setting the gyro-angle; instead, you just rotated the external mount by rather simple mechanical gear. It was also pretty handy in self-defense - destroyer, charging to drop depth bombs on French submarine would clearly be worried about the possibility of being greeted with torpedo salvo. Most of French submarines have trainable tubes that could only be reloaded in base, but "Surcouf" actually have reloadable torpedo mounts.

It also helps compensate for the lack of manoeuvrability in a large submarine like this. Some British submarines also had trainable mounts.

It is off-topic - but you might want to look up the pre-WWI Drzewiecki 'drop collars' which would actually hinge outward to fire the (essentially externally mounted) torpedoes at a chosen angle. In many cases the majority of the torpedo armament was carried in this way.

It is off-topic - but you might want to look up the pre-WWI Drzewiecki 'drop collars' which would actually hinge outward to fire the (essentially externally mounted) torpedoes at a chosen angle. In many cases the majority of the torpedo armament was carried in this way.

Yep, I knew about them.

Were rather popular in Russian & French navies before and during WW1, but eventually fell out of use, since they were less accurate than internal tubes, and externally-mounted torpedoes tended to be less reliable due to deterioration. Also, there were a lot of worries about externally-mounted torpedo exploding under enemy fire on surface (or from depth charge shockwave).

The Surcouf's were a generation much more modern though, with the torpedoes in sealed tubes on a turntable. That was popular with the French and also used by Britisj and Italians (/Swedes) and probably others. The French still used it post war although often fixed external tibes in the casing (per British WW2).